Cast nickel-base alloy

ABSTRACT

There is disclosed herein a cast nickel-base alloy having outstanding utility in gas turbine engine applications, particularly as directionally solidified, at a composition consisting essentially of, by weight, 17.5-18.5% molybdenum, 7.75-8.25% aluminum, up to 0.05% carbon, balance nickel.

United States Patent [54] CAST NICKEL-BASE ALLOY [50] Field of Search 75/170, 171; 148/32,32.5,1.6, 162

[56] References Cited UNITED STATES PATENTS 2,542,962 2/1951 Kinsey 75/170 Primary Examiner-Richard 0. Dean AttorneyRichard N. James ABSTRACT: There is disclosed herein a cast nickel-base alloy 2 CIaimS3 Drawing Figs having outstanding utility in gas turbine engine applications, [52] US. Cl 148/325, particularly as directionally solidified, at a composition con- 75/170, 148/162 sisting essentially of, by weight, l7.5-18.5% molybdenum, [51] Int. Cl C22c 19/00 7.758.25% aluminum, up to 0.05% carbon, balance nickel.

Ni M ,4Z

STRESS RUPTURE CAPAB/L/TV A5 lNFLUENCED B) At a Mo CONTENT, /800F /5,000PS/ o 8.0 %AA /0.0 7,41 100- PATENTEU NUV2 |97l AVERAGE RUPTURE L/FE HOURS SHEET 1 BF 3 N/ --MQ AZ STRESS RUPTURE CAPAB/L/TV AS lNFLUENCED 8) AZ & Mo CONTENT, /800F /5,000 P5 Z0 AL I I I l I fi O 4 8 I2 I6 20 24 WEIGHT M0 INVENTOR OUGLAS H. MAXWELL BY M/ W5.

ATToR Y PATENTEU nave m SHEEI 3 OF 3 v wmbex mit ON- ON a 18 .56 Q mo ES 38m. ME 6 3R X3538 #EREQB .8 GEE CAST NICKEL-BASE ALLOY BACKGROUND OF THE INVENTION The present invention relates in general to the cast nickelbase alloys. particularly those adapted to high-temperature service in gas turbine temperature applications. It further relutes to articles produced from such alloys by directional solidification techniques.

In the design of the more advanced gas turbine engines one of the principal limitations imposed on the designer in terms of providing increased engine performance and efficiency are the high-temperature strength limitations of the available engine alloys. Even in todays operational engines, the various alloys utilized are frequently exposed under high tress to temperatures in excess of 85 percent of their melting points. Accordingly, an urgent need exists for new materials which will not only provide improved high-temperature strength in engine environments, but which will also display such characteristics as oxidation, fatigue and creep resistance.

In the prior art U.S. Pat. to Kinsey No. 2,542,962, there is described a series of alloys within the broad compositional range of, by weight, l35 percent molybdenum, tantalum, tungsten and/or columbium, 2.4-l2.l percent aluminum, up to 0.15 percent carbon, balance essentially nickel. It has now been discovered that, within the broad range disclosed by Kinsey, there exists a unique alloy displaying useful properties unexpectedly superior to those of the currently available alloys. As hereinafter discussed in detail, the compositional limits of this new alloy are so critical that any significant variation therefrom results in a totally unsatisfactory material for high-temperature engine use. Indeed, because of the narrowness of the compositional limits involved in this new alloy, it is not surprising that its remarkable properties have not previously been discovered. Despite a similarity in overall chemistry to the prior art alloys, it is only within the narrow compositional limits disclosed herein that the particular advantageous metallurgical structure is achieved as clearly evidenced not only by the startling difference in mechanical properties of this alloy but also by its metallurgical uniqueness as evidenced by metallographic examination.

SUMMARY OF THE INVENTION The present invention contemplates a cast alloy at the basic composition of, by weight, l7.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, up to 0.05 percent carbon, balance nickel. In this alloy tantalum may be included on a substitutional basis for the molybdenum in an atomic ratio of up to about 1/1. Similarly, tungsten may be included in this alloy on a substitutional basis for the molybdenum up to about 16 weight percent tungsten based on the total weight of the constituents, although tungsten drastically decreases the fabricability of the alloy and increases its specific weight and, thus, in many instances is not preferred. The total tantalum and tungsten substitutions, if made, should be limited to about weight percent of the total weight. Still further, the alloy is receptive to the addition of reactive metals, such as yttrium, scandium and lanthanum, which are often utilized to foster 0X- idation-erosion resistance, in weight percentages up to about 1 percent.

In a particular preferred embodiment of the invention as conventionally cast yielding either a substantially equiaxed grain structure, or castings in columnar-grain or monocrystal form the composition comprises, by weight, l7.5l8.5 percent molybdenum, 7.75-8.25 percent aluminum, 0.01-0.05 percent carbon, up to 0.20 percent manganese, up to 0.20 percent silicon, up to 0.015 percent sulfur, up to 0.35 percent iron, up to 0.10 percent copper, balance essentially nickel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph illustrating the stress-rupture capability of the cast alloys of the nickel-molybdenum-aluminum system, at 1,800" F. and 15,000 p.s.i., as a function of their molybdenum and aluminum content and relationship.

FIG. 2 is a graph comparing the alloy of the present invention withthe conventional MAR M200alloy, as directionally solidified, in creep at various temperatures.

FIG. 3 dramatically illustrates the efi'ect of directional solidification on the 2,200 F. creep of the nickel-molybdenum-aluminum alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As hereinafter used in the description, the alloys of the present invention are often referred to as the Ni-l8Mo-8Al alloys.

At a nominal composition of 18 weight percent molybdenum, 8 weight percent aluminum, balance essentially nickel, and within the ranges of 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, up to 0.05 percent carbon, balance essentially nickel, the alloys of this invention comprise a dispersion of the strengthening 'y' nickel aluminide precipitate, Ni Al, at about volume percent, in a solid solution of nickel and molybdenum.

The extreme criticality and interrelation of the molybdenum and aluminum components of this alloy are dramatically illustrated in FIG. 1, and still further in the following table.

As clearly evidenced by the data, the strength of the present alloy is profoundly influenced by the amount of molybdenum and aluminum present in the alloy particularly in combination. At low aluminum levels, the high-temperature capabilities of the alloy are adversely afiected with dramatic suddenness so that at 2,200 F. the alloy is completely lacking in any useable strength. This is unusual in the sense that a relative minor reduction of the low melting component drastically influences its high-temperature strength. Actual analysis of the phenomenon reveals that, at the lower aluminum levels the 'y' precipitate is solutioned at lower temperatures.

At the higher aluminum contents, 9 percent by weight for example, the strength of the alloy is suddenly and markedly reduced, as compared to the alloy of the present invention, at all temperatures. The extent of the sudden strength decrease associated with such a small increase in aluminum content is believed to be a direct function of the conversion of at least a portion of the high strength 7" aluminide, Ni;Al to the lower strengthfl aluminide, NiAl.

With respect to the molybdenum content of the alloy it is evident, particularly after reference to FIG. 1, that extreme criticality exists in terms of composition and particularly insofar as the molybdenum/aluminum relationship is concerned. At molybdenum contents lower than those associated with the present invention, there is insufficient molybdenum present for adequate solid solutioning strengthening. At the higher molybdenum contents the alloy is susceptible to the formation of brittle intermetallics which prove fatal to the alloy insofar as gas turbine engine applications are concerned.

Carbon, or the absence of carbon, in the present alloy also assumes a rather critical role, particularly in columnar-grain or monocrystal form. In those components formed by conventional casting techniques to provide substantially equiaxed microstructures, carbon in the range of about 0.03-0.05 weight percent is advantageously included to provide creep and tensile ductility. In the directionally solidified components, however, the carbon content is necessarily limited to minimize the occurrence of the M C-type carbides. These carbides, which assume considerable size in directional solidification casting because of the relatively slow solidification rates TABLE 1v normally associated therewith, have been formed to be subject to precracking during solidification or to initiate failure sites RT 3,, Hardness after Heat Treatment for Alloys A, B, C, and D for cracking during use. Inasmuch as in the columnar-grain or Condition monocrystal form of the alloy, grain boundaries are either 2,150 F. 2,100 F. aligned in the direction of applied stress or are virtually nonex- 5 A5 cast (8) Q (1) 0Q istent, the presence of carbon for creep and tensile ductility is Ni-12Al-18Mo 36.0 35. 0 36.; not required. in one series of tests, involving 24 directionally fiijfigfifij; 2%: 5%,? 28: solldlfied bars 1n two heats, F-3367 and F3368,the follow- N1-8A1-12M0 27. 2 28. 6 34. 0 ing observations were made. The actual chemical analyses of theseheats as determined by the supplier were as follows: 10

TABLE V Efiect of Heat Treatment on Ni-18Mo8A1 Heat Al(wt.%) M0(wt.%)' C(wt.%) Ni Tensile properties Temp, 0.2% YS, UTS, Percent -3367 7.57 17.7 0.01 551. Condition F. K s.i. K s.i. elon. 54358 6.42 18.0 0.006 1121. AS 0 H 1' 800 73. 0 84 7 4 5 2,250 F. (4) 0Q- 1, 800 91.6 102.5 4.0 The difference in chemistry between the two heats, 77 01mm 800 5 although slight, resulted in a difference in microstructure. 20; Stress rupture properties Bgth alloy compositions co ntainerili aA 1r1)icll el eutecltidc-tylpe Test S Hows P t p ase 1n a reglon 0 7 plfiClpltgte 1 p us a y SO 1 so utengp tress, to ercen tion (Ni, Mo, Al), the eutectic-type phase being identified by F K electron microprobe analysis as having a composition of As cast 2,000 8.0 28.3 4. 4 Do 2, 000 8. 0 37. 4 1. 7 about, by weight, 39 percent molybdenum, 4 percent alu- 23005 (1) 2, 000 3, 0 32A minum, balance nickel. However, metallurgically speaking a 0 2,000 8. 0 31.5 2.8 2,000 .0 27.5 7.8 there was a notlceable difference between the two heats as 77 F [hr coolirom 25000 F 8 evidenced by the almost complete absence of the eutectictype phase in the F3368 (low aluminum) alloy. VI It is, of course, evident from the data that the properties of TABLE these alloys are drastically influenced by the alloy composition As Cast Imp Data for N1-18M0-8A1 and within very precise compositional limits. This is the direct Charpy Charm, result of the fact that, despite somewhat closely, related smooth notched impact impact chemical composmons, the solidified articles at the various t1 stren tn, compositions are in fact distinctive entities metallurgically. 35 Test D- A number of alloy melts were made in a variety of composi- 50. 0 9.0 tions, the representative results of which are summarized 58g 9 0 below.

TABLE II Alloy Chemistry Alloy A Alloy 13 Alloy 0 Alloy D Element (wt. percent) Aim Actual Aim Actual Aim Actual Aim Actual Balance TABLE III As Cast Mechanical Properties of Four Nl-Mo-Al Compositions As east tensile properties As cast stress rupture properties Be as cast Test temp, 0.2% YS. UTS, Percent Test temp., Stress, Hours to Percent Alloy hardness F. K s.i. K 5.1. 01011. F. K s.i. rupt. elon.

70 85.0 115.0 2. 0 70 87. 7 116. 0 2.0 1,000 112. 9 127. 6 2. 0 Ni12Al18Mo 36. 0 1, 400 86. 5 109. 0

1,800 57. 2 3.0 2,000 26. 3 28.1 5. 5 2,000 23.1 5. 0 70 75. 3 124. 4 4,0 70 77.2 122.2 Y 3.5 1, 000 98. 2 134. 5 3. 5 Nl-10Al-18Mo 32. 2 1, 400 94. 4 116. 8 2. 5 1, 800 59. 5 66. 4 3. 0 2, 000 83. 7 41. 7 7. 5 2, 000 30. 8 41. 6 7. 0 70 102. 0 163. 6 3. 5 70 102. 6 140. 2 8. 0 1.2 1112 123-2 Ni-8Ai-18Mo 37. 8 800 0 7 2,000 49. 0 68.3 5.0 2,000 56. 9 64. 3 5. 5 2,200 16.8 25.4 15.0 70 79. 6 143. 0 17. 5 13-: 111-: 2-2 1:400 1055 10818 Alloy C exhibited the best overall properties with useful tensile and stress-rupture strengths to 2,200 F. Alloys A, B and D were not considered satisfactory due either to low rupture strength and/or ductility. Alloy C further showed a major response to homogenization heat treatment at 2,250-2,300 F. in that elevated temperature tensile strength was significantly improved over theas-case properties. The stress-rupture properties on the other hand did not appear to be influenced by a high homogenization heat treatment. Alloy C further showed excellent potential for hardfacing applications since its hardness after a drastic cooling was observed to be significantly higher than its as-cast hardness. In summary, the results indicate that the Ni-18Mo-8Al alloy is unique, has outstanding tensile strength in the 1,400-2,200 F. range; and useful stress-rupture strength in the 2,000-2,200 F. range.

Testing of the alloy in the as-cast or equiaxed condition revealed that both rupture strength and ductility were controlled primarily by the ability of the grain boundaries to resist fissuring and sliding. 1n the NH 8Mo-8A1 alloy only a fraction of the creep damage incurred during testing could be attributed to plastic deformation within the grain itself. Accordingly, although the alloy has superior properties in the equiaxed condition, the true strength of the alloy could not be approached until all grain boundaries not parallel to the stress axis. This was accomplished by directional solidification of the alloy to produce a columnar grain structure in which the existing grain boundaries were aligned with the axis of applied stress. In directional solidification testing performed with stock from the F3367 and F-3368 heats previously mentioned, the effect of directional solidification of the 2,200 F. creep of these alloys was observed, as set forth graphically in FIG. 3.

Additionally, 12 test bars were directionally solidified at the following chemistry in two separate heats:

This material, tested uncoated in air, displayed the tensile and creep rupture properties reported in tables VII and VIII.

TABLE VII Tensile Test Results Elong. RA UTS (p.s.i.) (percent) (percent) Temp., F 0.2 48 (p.s.i.)

TABLE VIII Creep Rupture Hours to emp. Stress Final elon- Coudition F.) (p.s.i.) 1% Rupture gation H.T 1, 400 85,000 52. 3 1. 9 H.T. 1, 800 5, 000 110 394. 7 14. 5 H.T 1,800 20,000 41 105.4 10. 5 H.T 1,800 5,000 5 21.8 11.4 H.T 1, 900 22, 500 3 9. 0 15.6 H.T- 2, 000 9, 000 84 191. 8 10. 0 AC. 2, 000 13,000 5 35. 5 36. 2 H.T. 2, 000 13, 000 27 49. 3 12. 6 H.T 2, 8, 000 35 186. 5 8. 1 H.T 2, 100 10, 000 43 107. 5 8. 5 H.T 2, 200 5, 000 55 109. 5 11. 9 H.T 2, 200 6,500 12 33. 7 16. 4

NOTE. H.T.:Heet treatment (argon) 2,300 F. /2 Hr., air cool rate.

A comparative summary of various materials in several metallurgical forms is reported in the following table.

In a supplemental program, the Ni-l8Mo-8Al was tested in a coated condition to determine if either the conditions under which the coating was applied or a subsequent diffusion of coating elements would have any deleterious effect on the alloy substrate. Coated specimens schromium modified aluminide) tested at 1,800 F and 2,000 F. revealed no detrimental effects from the coating utilized and, in fact, exhibited improved lifetimes.

In view of the foregoing and within the specific compositional limits set forth, the alloy of the present invention may be seen to possess a uniqueness which renders it particularly suitable for high temperature applications in the advanced gas turbine engines. Its obvious superiority over the currently utilized alloys immediately suggests such use. However, it will be understood that its utility is not so limited, and its use in other applications such as hardfacing, or high-temperature mold or die construction is suggested.

What is claimed is:

1. A nickel-base alloy directionally solidified casting of the columnar-grain or monocrystal type for use at temperatures up to 2,200 F which consists essentially of, by weight, 17.5-18.5 percent molybdenum; 7.75-8.25 percent aluminum; up to 0.05 percent carbon; up to 1 percent yttrium, scandium or lanthanum or mixtures thereof; up to 25 weight percent tantalum plus tungsten on a substitutional basis for molybdenum, the tungsten content not exceeding about 16 weight percent based on the total weight of the alloy, the tantalum content not exceeding 50 atomic percent based on the molybdenum content of the alloy, balance nickel.

2. A nickel-base alloy directionally solidified casting of the columnar-grain or monocrystal type which consists essentially of, by weight, 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, up to 0.05 percent carbon, balance nickel. 

2. A nickel-base alloy directionally solidified casting of the columnar-grain or monocrystal type which consists essentially of, by weight, 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, up to 0.05 percent carbon, balance nickel. 